The present invention relates to air compressors. More particularly, the present invention relates to an improved screw-type air compressor.
Rotary screw-type air compressors generally include a pair of complementary rotors mounted within an internal working chamber of the compressor housing. Each rotor has a shaft supported for rotational movement by a pair of opposed radial bearings. Air enters through an airend inlet and is compressed by the rotating rotors as it moves toward a discharge port at the discharge end of the chamber. The spacing between the end surfaces of the rotors and the discharge end face of the housing is referred to as the discharge end clearance. This discharge end clearance has a substantial effect on the performance of the compressor. Accordingly, it is desirable to precisely set and maintain an operating discharge end clearance of a given air compressor to achieve a desired performance.
Current methods of mounting the rotors with a desired operating end clearance generally require extensive, very precise machining of the rotors and the housings. Bearings must also be accurately manufactured to provide not only radial support, but also axial support. Even with precise machining, the desired end clearance is often not achieved without extensive assembly procedures, for example, precision measuring and calculating of relative housing and rotor assembly measurements and the inclusion of compensating components, including shim plates or like. In addition to precise machining and assembly, other factors, for example, the internal rotor gas forces, must also be calculated and compensated for.
The present invention provides an air compressor assembly of the rotary screw type that provides accurate discharge end clearances with minimized manufacturing and assembly requirements. The air compressor assembly comprises a housing having an internal working chamber that extends within the housing and terminates in a discharge end face at the discharge end of the housing. At least one rotor is mounted for rotation and axial movement within the working chamber. The rotor has a discharge end surface having a step defined thereon. The step is preferably machined to a height precisely equal to the desired discharge end clearance. A thrust piston extends from the rotor and is positioned within a thrust piston chamber. A pressure source is associated with the thrust piston chamber and is controllable between a high pressure condition and a reduced pressure condition. In the high pressure condition, a high thrust pressure is created such that the thrust piston is moved axially toward the discharge end and the rotor step abuts the housing discharge end face to precisely position the rotor with the desired discharge end clearance. This condition is generally referred to as the xe2x80x9cloadedxe2x80x9d condition during which the airend generally delivers compressed air to the intended application. In the reduced pressure condition, the thrust pressure is reduced and the rotor step moves away from the discharge end face to allow the rotor to freewheel. This condition is generally referred to as the xe2x80x9cunloadedxe2x80x9d condition during which compressed air is not delivered to the intended application by the airend.
A method of mounting a rotor with a desired end clearance in accordance with the present invention is also provided.